(65a) Multi-Layer Contactless Dielectrophoresis Using Thin Polyimide Films

Multilayer Contactless
Dielectrophoresis Using Thin Polyimide Films

Dielectrophoresis
(DEP), the motion of particles in a suspending medium due to their polarization
in a non-uniform electric field, is an effective means of microfluidic particle
manipulation. Traditionally, DEP has been performed using electrodes patterned
in the bottom of a sample channel, and has been successfully utilized for a
variety of applications, ranging from separating live and dead cells to
trapping cells, viruses, and DNA, to determining antigen expression. However, challenges
in using this technique include Joule heating, bubble formation, and electrode
delamination. Contactless dielectrophoresis (cDEP) addresses these issues by
using conducting fluid electrodes in side channels along a central sample
channel, separated from the sample channel by a thin insulating barrier. Metal
electrodes are inserted into the conducting fluid and the electric field is
then applied across the sample channel without direct contact between the
electrodes and the sample. When applying an electric field across the device,
there is a large voltage drop across the insulating barrier and a relatively
small drop across the sample channel itself. In order to achieve a sufficient voltage
drop across the sample channel, much larger voltages must be applied across the
device. The maximum applied voltage is limited by instrumentation capabilities
and the dielectric breakdown properties of the barrier material.

The
insulating barrier behaves as a resistor and capacitor in parallel and analysis
shows that a reduction in resistance and an increase in capacitance of the
barrier decreases the voltage dropped across this barrier and enables cDEP devices
to operate over a larger frequency spectrum. For mammalian cells with similar
phenotype, the largest difference in the Clausius-Mossotti factor occurs close
to their first crossover frequency. In low conductivity physiologically
relevant buffers, this is usually between 5-100 kHz, a range reaching below the
lower frequency limit of some previous single layer PDMS cDEP devices.

A
multilayer device in which the fluid electrodes and sample channel are in
separate stacked layers can help achieve a reduction in the voltage drop across
the insulating barrier. Increasing the area of the electrode-sample channel
interface reduces the resistance while increasing the capacitance of the
barrier. In single-layer devices, this interfacial area is limited by the depth
of the sample channel, which in turn is limited by the depth of the silicon
wafer stamps whether using deep reactive ion etching (DRIE) or photoresist
spin-coating (such as SU-8). However, a multilayer device enables the fluid
electrode area to be increased by locating it above the sample channel; it is
then limited only by the specified channel width.

In
this work, we capitalized on the observation that the voltage drop from fluid
electrode to sample channel can be further decreased by choosing a barrier
material that has a higher relative permittivity than PDMS. Here, a commonly
available thin polyimide tape was utilized as the barrier between the fluid
electrode and sample channel. The use of a higher dielectric constant
self-adhesive thin polyimide film with a multilayer cDEP device provides a
simple and cost-effective means of significantly increasing the potential of
cDEP for sorting of mammalian cells.

Master
stamps were created for the fluid electrodes and sample channel on separate
silicon wafers using deep reactive ion etching. Polymer replicates were then
cast in PDMS with a 10:1 ratio of polymer to curing agent. After punching
access holes to each of the fluid channels, a strip of polyimide tape was
applied to the PDMS replicate containing the fluid electrode channels. The
fluid electrode and sample channel substrates were bonded together after
exposure to air plasma for two minutes such that only the electrode cannel was in
contact with adhesive. The completed devices were stored under vacuum.

Prior
to experimentation, the fluid electrode channels were filled with phosphate
buffered saline with a conductivity of 1.4 S/m. Polystyrene microspheres were
suspended in either DI water or a low conductivity sucrose solution with
conductivities of 0.001 and 0.01 S/m, respectively. A high voltage amplifier
and series transformer were used to apply voltages with amplitudes up to 300 V_RMS
between 10 and 800 kHz.  Preliminary experiments match theoretical predictions
that these multilayer devices with polyimide film barriers induce a DEP
response in particles over a wider frequency spectrum than single layer devices
with PDMS barriers.

This
work presents a simplified process for fabricating multilayer cDEP devices. The
use of manufactured polyimide films ensures uniform thickness of the insulating
barrier between batches and provides superior electrical performance compared
to PDMS. The use of an adhesive film eliminates the need for multiple bonding
steps and the multilayer fabrication process allows for the use of geometries which maximize the frequency response of these devices.